1. Field of the Invention
The invention relates to a guard ring which in rapid heat treatment of a round blank-like substrate, such as a semiconductor wafer or the like, using a heating device of the light irradiation type, fixes the substrates in a horizonal position and heats the outside circumferential surface in addition by radiation.
2. Description of Related Art
A round blank-like substrate, such as a semiconductor wafer, is subjected to heat treatment for layer formation, diffusion, baking and the like which comprises rapid heating, holding at a high temperature, and rapid cooling. Using a heating device of the light irradiation type a temperature increase to greater than or equal to 1000.degree. C. can be achieved in a few seconds and rapid cooling produced by stopping the light irradiation.
The temperature in the peripheral area of the semiconductor wafer, as a result of heat irradiation from the outer circumferential surface of the semiconductor wafer or for similar reason, is lower than in its middle area, even if the semiconductor wafer surface is uniformly irradiated with light. In the case, for example, in which the temperature of the middle area of the semiconductor wafer is 1100.degree. C., the temperature in the peripheral area is about 30.degree. C. lower than in the middle area. If, in this way, the middle area and the peripheral area of the semiconductor wafer have a temperature difference, and thus, the temperature distribution becomes nonuniform, there are cases in which a dislocation error occurs which is called "slip," and in which scrap is produced.
To prevent formation of slip in the semiconductor wafer, a guard ring is used to prevent formation of the temperature difference between the middle area and the peripheral area of the semiconductor wafer during its heat treatment by rapid heating, holding at a high temperature, and rapid cooling. This guard ring is an annular body which is formed of a thin plate and of metal with a high melting point, such as molybdenum, tungsten or tantalum, or of a ceramic such as silicon carbide or the like. In the inner peripheral area of its circular opening, a semiconductor wafer bearing part is formed. The guard ring surrounds the semiconductor wafer.
This means that the guard ring is arranged such that the semiconductor wafer is installed in the circular opening of the annular guard ring. The bearing part holds the semiconductor wafer. Due to the light irradiation, the guard ring itself reaches a high temperature and also heats the opposite outer peripheral edge area of the semiconductor wafer by radiation, compensating for the heat radiation from the outer circumferential surface of the semiconductor wafer. In this way, the temperature difference between the middle area and the peripheral area of the semiconductor wafer is reduced, the temperature distribution is made essentially uniform, and the formation of slip is prevented.
One such guard ring is disclosed, for example, in the Japanese patent disclosure document HEI 9-22879.
As was described above, in the circular opening of an annular guard ring a semiconductor wafer is installed and the guard ring itself, due to light irradiation, reaches a high temperature. In this way, the heat radiation from the outer circumferential surface of the semiconductor wafer is balanced. To achieve suitable compensation (neither too much nor too little) and to eliminate the temperature difference between the middle area and the peripheral area of the semiconductor wafer, a state is desired in which the guard ring is a nearby semiconductor wafer and can act more or less as the outer edge area of the semiconductor wafer. This means that it is preferred that the height of the inner circumferential surface of the circular opening of the guard ring is equal to the height of the outer circumferential surface of the opposite semiconductor wafer, the thickness of the inner circumferential surface of the circular opening of the guard ring is likewise equal to the thickness of the outer circumferential surface of the opposite semiconductor wafer and also the heat capacity per unit of area is as great as the heat capacity of the wafer per unit of area. When one such guard ring is used, during light irradiation a temperature change of the guard ring exactly following the temperature change of the semiconductor wafer can be achieved. Therefore, the heat radiation from the outer circumferential surface of the semiconductor wafer can be exactly compensated and a uniform temperature distribution obtained.
If the heat capacity of the guard ring per unit of area is large, the temperature increase of the guard ring is delayed, by which the amount of compensation with regard to heat radiation from the outer circumferential surface of the semiconductor wafer becomes less as the temperature increases. Here, the temperature distribution of the semiconductor wafer cannot be made uniform. Furthermore, when the temperature drops after stopping the light irradiation, the temperature drop of the guard ring is slowed down. In this case, compensation with regard to the heat radiation from the outer circumferential surface of the semiconductor wafer is unduly high so that the temperature distribution of the semiconductor wafer cannot be made uniform. When the heat capacity of the guard ring per unit of area is small, the exactly opposite phenomena occurs, and likewise the temperature distribution of the semiconductor wafer cannot be made uniform.
Since the semiconductor wafer is installed in the circular opening of the guard ring, with consideration of workability during installation, variance of the dimension of the outside diameter of the semiconductor wafer, and the difference of the thermal expansion between the guard ring and the semiconductor wafer, clearance of about 1 mm between the inner circumferential surface of the circular opening of the guard ring and the outer circumferential surface of the semiconductor wafer is required.
In the case of determining the temperature of the semiconductor wafer with a radiation thermometer of the noncontact type from the bottom (rear surface) of the semiconductor wafer, therefore, an error occurs in temperature measurement when from this clearance light enters, making it difficult to control the temperature of the semiconductor wafer. As a result, light must be prevented from entering through the clearance between the inner circumferential surface of the circular opening of the guard ring and the outer circumferential surface of the semiconductor wafer.
When supporting the semiconductor wafer by means of the guard ring it is necessary to make the contact surface as small as possible and to reduce the heat conduction between the two. If the temperature change of the guard ring exactly follows the temperature change of the semiconductor wafer, only little heat conduction occurs even if the contact surface is large. However, exact matching is difficult in practice. If the contact surface is large, heat transport due to heat conduction occurs between the guard ring and the semiconductor wafer. If heat conduction occurs, the temperature of the semiconductor wafer is influenced by the temperature of the guard ring, making computation for prediction of the temperature distribution of the semiconductor wafer very complicated and thus also complicating control to make the temperature of the semiconductor wafer uniform. If heat conduction can be prevented, it is enough only to consider heat radiation, simplifying the prediction and control of the temperature distribution.
For the material of the guard ring, there is a need for properties such as excellent heat resistance, excellent acid resistance and good thermal conductivity and the like. A material which has exactly these properties is ceramic, such as silicon carbide (SiC) or the like. But, it is difficult to process these ceramics with high precision. If they have complicated or thin shapes, they are difficult to produce. Therefore, t is necessary for them to have shapes which facilitate processing.
FIGS. 7 to 9 each show a conventional example of a guard ring. Here, the guard ring disclosed in the Japanese patent disclosure document HEI 9-22879 and described above is shown in cross section.
In FIG. 7, a guard ring 26 (an auxiliary plate similar to the substrate) has three small projections 30 (only one of which is shown) in the form of thinned plates projecting in a direction toward the middle from the inner circumferential surface of a main part 28 of a thin plate with the same thickness as that of a semiconductor wafer W. The bottom of the main part 28 of the thin plate and the bottoms of the projections 30 are in the same plane. The semiconductor wafer W is supported by the projections 30. Since the bottom of the main part 28 and the bottoms of the projections 30 are in the same plane, the height of the inner circumferential surface of the main part 28 which is opposite the outer circumferential surface of the semiconductor wafer W is lower. Therefore, the heat radiated from the outer circumferential surface of the semiconductor wafer W is not adequately compensated. Furthermore, it can be considered a disadvantage that heat conduction occurs between the semiconductor wafer W and the guard ring 26 because the semiconductor wafer W is in surface contact with the projections 30.
In FIG. 8, a guard ring 36 is shown, and in it, there are three small projections 40 in the form of thinned plates projecting toward the middle from the inner circumferential surface of a main part 38 of a thin plate with the same thickness as that of a semiconductor wafer W as in the guard ring 26 in FIG. 7. The projections 40 project from the bottom of the main part 38. The bottom of the main part 38 and the tops of the projections 40 are in the same plane. Therefore, the outer circumferential surface of the semiconductor wafer W is as high as the inner circumferential surface of the opposite main part 38, by which the heat radiated from the outer circumferential surface of the semiconductor wafer W can be adequately compensated. However, since the semiconductor wafer W is in surface contact with the projections 40, heat conduction occurs between the semiconductor wafer W and the guard ring 36. To reduce the amount of heat moving by this heat conduction, the thickness of the projections 40 must be reduced to reduce the heat capacity. But, it is difficult to make projections 40 in the form of very thin plates.
In FIG. 9, a guard ring 46 is shown and in it there are three small projections 50 in the form of thinned plates projecting toward the middle from the inner circumferential surface of a main part 48 of a thin plate with the same thickness as that of a semiconductor wafer W. The tops of the projections 50 are provided with microscopically small convex sites 52 which support the semiconductor wafer W by point contact. Thus, the amount of heat moving by heat conduction is reduced. But, in this case as well, the height of the inner circumferential surface of the main part 48 which is opposite the outer circumferential surface of the semiconductor wafer W is lower because the bottom of the main part 48 and the bottoms of the projections 50 are in the same plane. Therefore, the heat radiated from the outer circumferential surface of the semiconductor wafer W is not adequately compensated. Furthermore, it is very difficult to form convex sites 52 on the tops of the projections 50.
The guard rings as shown in FIGS. 7 to 9 have the aforementioned defects. In each of these guard rings, light from the clearance between the outer circumferential surface of the semiconductor wafer W and the inner peripheral surface of the main part of the thin plate enters by diffraction towards the bottom because, for example, three projections in the form of thinned plates support the semiconductor wafer W. In the case of determining the temperature of the semiconductor wafer using a radiation thermometer of the noncontact type from the bottom of the semiconductor wafer, therefore, an error occurs in temperature measurement due to this light which enters from this clearance by diffraction. This results in the disadvantage that it is difficult to control the temperature of the semiconductor wafer.
These guard rings are formed from a ceramic material such as silicon carbide or the like. However, it is very difficult processing to form small projections in one piece and projecting upwards in the form of thinned plates on the inner circumferential surface of the main part of the thin plate which has been formed from ceramic, i.e. which is "pottery."